Superparticles (SPs) or colloidal superparticles (CSPs) are assemblies of crystalline structures that include a plurality of nanoparticles arranged in a primary and/or a secondary hierarchical structure to form the SPs. SPs combine the intrinsic physical characteristics of their nanoparticle building blocks including a large surface to volume ratio and higher surface area. SPs couple the properties of individual nanoparticles to exhibit new collective properties. The shape and size of SPs can be tuned by varying the shape, size, and composition of their nanoparticle building blocks.
Conventional strategies for synthesizing colloidal superparticles (SPs) generally rely on self-assembly of pre-synthesize nanoparticles with well-controlled size, size distribution, shape, composition and surface chemistry. The driving force for self-assembly originates from solvent evaporation, polymer templating, electrostatic interaction, hydrogen bonding, and/or interfacial tension. Such self-assembly methods can include induced solvophobic interactions and micro-emulsion templating to enable self-assembly of nanoparticle building blocks into desired SPs.
Conventional self-assembly methods have found great success in synthesizing SPs made of a large number of nanocrystals with different shapes and compositions. While such SPs exhibit significantly enhanced performance, for example enhanced magnetic and optical properties relative to individual nanoparticles, the high surface area, which is considered as the most important property of the nanoparticles, no longer exists in such conventional SPs. This is because conventional self-assembly methods use surfactants, which can include organic ligands, during SP synthesis. The organic ligands that coat the surface of the nanoparticle building blocks are critical for conventional assembly of the nanoparticles into SPs, and occupy the gaps between the individual nanoparticle building blocks in SPs. The presence of surfactants reduces the accessible surface area in the conventional SPs relative to the overall surface area of the freestanding building block nanoparticles.
Although the organic ligands or otherwise surfactants can be removed through calcination of the SPs at elevated temperature, such high temperature processes are detrimental to the stability and mesoscopic structures of the SPs. Alternatively, deposition of surfactant molecules on the surfaces of individual SPs can be avoided by integrating the synthesis of individual nanoparticles and their aggregation into SPs in a single step. However, such one step synthesis process yield SPs including clean surfaces which always undergo entropy-driven aggregation to coalesce and fuse together to reduce the surface energy as well as the surface area.